Effect of gravity on density distributions and orthopositronium annihilation rates in ethane and methane near the critical point

Abstract

The effect of gravity on density distributions has been studied in ethane and methane near their critical points using the linear-model parametric equation of state. The results obtained from this study are used to further understand the sensitivity of orthopositronium annihilation rates to density fluctuations in molecular gases. It is shown that the influence of gravity is too small to account for the recently observed dependence of orthopositronium annihilation rates on the density of ethane gas at 306.4 K. However, a significant variation in local density vs height is calculated at temperatures closer to the gas–liquid critical point. The density and temperature dependencies of the annihilation rates of orthopositronium atoms, recently observed in ethane and methane gases, are discussed in terms of the findings of this study.